• Cranial Nerve I: Olfactory Nerve
• Cranial Nerve II: Optic Nerve
• Cranial Nerve III: Oculomotor Nerve
• Cranial Nerve IV: Trochlear Nerve
• Cranial Nerve V: Trigeminal Nerve
• Cranial Nerve VI: Abducens Nerve
• Cranial Nerve VII: Facial Nerve
• Cranial Nerve VIII: Vestibulocochlear Nerve
• Cranial Nerve IX: Glossopharyngeal Nerve
• Cranial Nerve X: Vagus Nerve
• Cranial Nerve XI: Accessory Nerve
• Cranial Nerve XII: Hypoglossal Nerve

1. What Is the Function of the Olfactory Nerve?

The olfactory nerve or cranial nerve I is the shortest cranial nerve and is involved in transmitting smell (olfaction) information to the brain for processing [1].

It originates from the olfactory epithelium covering the nasal cavities and where olfactory sensory neurons (bipolar neurons) are found and from which the olfactory fibers of the olfactory nerves originate and extend.

The olfactory nerves ascend and cross openings in the frontal bone of the skull known as olfactory foramina and connect with the olfactory bulb which sends olfactory information to the amygdala, hippocampus, and orbitofrontal cortex for further processing.

• What Happens When Your Olfactory Nerve Is Damaged?

Damage of the olfactory nerve can be caused by genetic defects, tumors, trauma, or surgery (iatrogenic) and can result in partial (hyposmia) or total loss (anosmia) of smell.

2. What Is the Function of the Optic Nerve?

The optic nerve is the 2nd cranial nerve responsible for the transmission of visual information from the retina to the brain such as colors, brightness, and contrast (acuity) [2].

The optic nerves are made of axons of the retinal ganglion cells that receive visual information from photoreceptors in the eye retina through intermediary neurons known as bipolar cells and retina amacrine cells.

Optic nerves (right and the left) leave the orbital sockets through openings in the middle front of the skull known as optic canals and reach the optic chiasma where they cross each other and continue their extension to the lateral geniculate nuclei.

At the optic chiasma, the optic nerve comes from the left eye extent towards the right hemisphere of the brain, while the optic nerve comes from the right eye extent towards the left hemisphere of the brain.

The lateral geniculate nuclei relay the visual information to other structures of the brain for further processing.

• What Happens When Your Optic Nerve Is Damaged?

Damage to the optic nerve results in a permanent loss of vision on the left, the right, or both fields of vision, depending on the type of damage which can affect one of the optic nerves or both.

Damages to the optic nerves can be due to the following conditions or disorders:

• Optic Neuritis
• Glaucoma
• Ischemic Optic Neuropathy
• Tumor Compression
• Inflammation
• Genetic or Congenital Diseases (e.g., Leber’s hereditary optic neuropathy)

3. What Is the Function of the Oculomotor Nerve?

The oculomotor nerve is the 3^rd cranial nerve involved in most movements of the eye and eyelids [3].

It is a motor nerve that originates from a cluster of neurons within the midbrain known as the nucleus and enters the orbit through the superior orbital fissure.

Once within the orbit, the oculomotor nerve branches into an inferior and superior branch.

The inferior branch innervates the medial rectus, the inferior rectus, and inferior muscles responsible for eye movements in different directions and plans.

An ending of the inferior branch also emerges into the ciliary ganglion which is a group of nerves found at the back of the eye and part of the parasympathetic nervous system.

The superior branch innervates the superior rectus (eye movement) and the levator palpebrae superioris (eyelid movement) muscles.

• What Happens When Your Oculomotor Nerve Is Damaged?

Damage of the oculomotor nerve can result in paralysis affecting eye and eyelids movements. The damage can be caused by trauma, demyelinating diseases, microvascular diseases, or compression of the nerve by a brain tumor.

4. What Is the Function of the Trochlear Nerve?

The trochlear nerve is the fourth cranial nerve involved in the innervation of the superior oblique muscle of the eye responsible for eye rotation [4].

It is originated from a nucleus found in the midbrain and enters the orbit through the superior orbital fissure.

• What Happens When Your Trochlear Nerve Is Damaged?

Damage of the trochlear nerve results in double vision (diplopia) and can be caused by head trauma, intracranial pressure (e.g., brain tumor, hemorrhage) that compress the nerve, viral infections, demyelinating diseases, or congenital defects.

5. What Is the Function of the Trigeminal Nerve?

The trigeminal nerve is the fifth cranial nerve and is divided into the ophthalmic nerve, maxillary nerve, and mandibular nerve [5].

The ophthalmic nerve (V1) is a sensory (afferent) nerve responsible for sensations from the eyeball, upper face, and anterior scalp.

The maxillary nerve (V2) is a sensory nerve responsible for sensations from the nasal cavity and sinuses, mid-face, palate, and maxillary teeth.

The mandibular nerve (V3) is both a sensory (afferent) and motor (efferent) nerve responsible for sensations from the chin, ear, temple, oral cavity, tongue, temporomandibular joint (TMJ), mandibular teeth, proprioception from muscles of mastication, and motor functions such as biting and chewing.

The ophthalmic nerve exits the skull through the superior orbital fissure

The maxillary nerve exits the skull through the foramen (hole) rotundum.

The mandibular nerve exits the skull through the foramen (hole) ovale.

After exiting the skull, the ophthalmic nerve, the maxillary nerve, and the mandibular nerve converge into the trigeminal ganglion.

From the trigeminal ganglion, a single root extends toward the pons of the brainstem where it branches toward the cerebellum and the medulla (part of the brainstem), and another toward the thalamus where the information is processed.

• What Happens When Your Trigeminal Nerve Is Damaged?

The most common manifestations of trigeminal nerve damage are trigeminal neuralgia, cluster headache, and Wallenberg syndrome.

Trigeminal neuralgia is caused by the mechanical compression of the trigeminal nerve such as compression by an aneurysm, a tumor, trauma, or arteriovenous malformation.

It mainly manifests with neuropathic pain with a burning or shock-like sensation. Trigeminal neuralgia is mainly treated with carbamazepine, but also with medications including lamotrigine, phenytoin, baclofen, gabapentin, pregabalin, and oxcarbazepine [6].

Cluster headache causes are unknown. It manifests with severe headaches on one side of the head.

Wallenberg Syndrome or Lateral Medullary Syndrome is due to the damage of the medulla (part of the brainstem) by a stroke (lack of blood supply). It manifests with a loss of sensations in one side of the body.

6. What Is the Function of the Abducens Nerve?

The abducens nerve is the sixth cranial nerve involved in the movement of the eyeball [7].

The abducens nerve originates from a cluster of neurons in the pons of the brainstem, known as the abducens nucleus, and enters the orbit through the superior orbital fissure to innervate lateral rectus muscle, responsible for lateral movements of the eye.

• What Happens When Your Abducens Nerve Is Damaged?

Damage to the abducens nerve can be caused by injury or compression (e.g., aneurysm) resulting in diplopia (double vision).

7. What Is the Function of the Facial Nerve?

The facial nerve is the seventh cranial nerve that has sensory and motor functions [8].

The sensory function is associated with conveying taste sensations (tongue), while the motor function involves the control of muscles responsible for facial expression (motor function) and the middle ear.

The facial nerve has also parasympathetic fibers that travel to the ganglia involved in the sensory innervation of the lacrimal, nasal, palatine, submandibular, sublingual glands.

The facial nerve originates from the pons of the brainstem and exits the skull through the stylomastoid foramen.

• What Happens When Your Facial Nerve Is Damaged?

Damage of the facial nerve can be caused by surgery, trauma, idiopathic Bell palsy, stroke, tumors, or granulomatous meningitis. It manifests with symptoms including facial paralysis, excessive salivation, dry eyes, and taste problems.

8. What Is the Function of the Vestibulocochlear Nerve?

The facial is the eighth cranial nerve involved in conveying sound (hearing) and balancing information to the brain [9].

The facial nerve has two nerve branches, the cochlear nerve, and the vestibular nerve.

The cochlear nerve is involved in transmitting auditory information from the cochlea to the cochlear nucleus in the medulla oblongata (part of the brainstem) for processing.

The vestibular nerve transmits sensory information received by the vestibular hair cells (inner ear) to the brain for the processing of body balance information.

• What Happens When Your Vestibulocochlear Nerve Is Damaged?

Damage of the vestibulocochlear nerve can be caused by direct trauma, tumors, infection, congenital malformations, and vascular injury.

It presents with symptoms including vertigo, tinnitus, nystagmus, false sense of motion, motion sickness, loss of balance, and sensorineural hearing loss.

9. What Is the Function of the Glossopharyngeal Nerve?

The glossopharyngeal nerve is the ninth cranial nerve that has sensory, motor, and parasympathetic functions [10].

It is originated from the medulla oblongata and leaves the skull through the jugular foramen (at the base of the skull) where it branches into the tympanic, tonsillar, carotid sinus nerve, stylopharyngeal, branches to the tongue, lingual branches, and a communicating branch to cranial nerve X (vagus nerve).

The sensory function is associated with conveying sensations from the oropharynx, posterior tongue, carotid body, and sinus.

The motor function is associated with the control of the stylopharyngeus muscle involved in elevating the pharynx and the larynx, and in the dilatation of the pharynx for swallowing.

The parasympathetic function of the glossopharyngeal nerve is involved in the innervation of the parotid gland (secretion of saliva).

What Happens When Your Glossopharyngeal Nerve Is Damaged?

Damage to the glossopharyngeal nerve can be caused by a tonsil tumor, glossopharyngeal neuralgia, injury, and surgery, resulting in a loss of taste at the posterior part of the tongue and difficulties swallowing.

10. What Is the Function of the Vagus Nerve?

The vagus nerve is the longest and tenth cranial nerve that has sensory, motor, and parasympathetic functions [11].

It originates from the medulla oblongata and leaves the skull through the jugular foramen where it divides into different branches that extend to innervate parts of the body.

The sensory function of the vagus nerve conveys sensations from the larynx, hypopharynx, heart, lungs, abdominal viscera, the epiglottic region, and the hypopharynx.

The motor function of the vagus nerve is involved in the control of the muscles of the pharynx and larynx responsible for phonation and swallowing.

The parasympathetic function is associated with the control of cardiac muscle, and muscles and glands of the anterior and middle parts of the gut.

• What Happens When Your Vagus Nerve Is Damaged?

Because the vagus nerve has many functions the effects are also multiple such as vasovagal syncope (sudden drop in blood pressure and fainting), gastroparesis (delayed gastric emptying), difficulty speaking, and swallowing.

11. What Is the Function of the Accessory Nerve?

The accessory nerve is the 11^th cranial nerve involved in the neck and shoulder movements through innervation of the cervical sternocleidomastoid and the trapezius muscles [12].

It is a motor nerve that has two parts, one part is originated from the nucleus ambiguous in the medulla and the other part known as the spinal accessory nerve is originated from neurons in the upper spinal cord [13].

The accessory spinal nerve exits the skull through the jugular foramen (base of the skull).

• What Happens When Your Accessory Nerve Is Damaged?

Damage of the accessory nerve can be caused by trauma, tumors, or surgery and results in neck and trapezius movement difficulties.

12. What Is the Function of the Hypoglossal Nerve?

The hypoglossal nerve is the 12^th cranial nerve involved tongue movement through innervation of the extrinsic and intrinsic muscles of the tongue [14].

It is originated in the hypoglossal nucleus in the medulla and leaves the skull through the hypoglossal canal.

• What Happens When Your Hypoglossal Nerve Is Damaged?

Damage to the hypoglossal nerve can be caused by trauma or tumors and results in weakness of the tongue muscles (tongue does not stick out straight).

Conclusion

Although cranial nerves have many functions associated with the sense, they have also parasympathetic functions such as their involvement in the control of the heart, lungs, and digestive organs.

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However, there are several steps before taste information reaches the gustatory cortex for processing:

• When a food is consumed, its breakdown in the mouth liberates chemical substances that interact with the tongue surface that contains raised bumps known as papillae.

The papillae contain taste buds that are specialized in detecting the types of taste through receptors known as gustatory receptors.

The gustatory receptors release neurotransmitters that activate sensory neurons (neurons involved in sensation) in proximity.

• The activation of the sensory neurons transmits (neurotransmission) the taste information (gustatory information) to facial and glossopharyngeal cranial nerves.

• Upon reception of the taste information, the nerves transmit it to a part of the brain known as the gustatory nucleus found in the medulla oblongata located in the lower part of the brainstem.

The medulla oblongata is responsible for several functions in the autonomous nervous system (involuntary nervous system), including swallowing, vomiting, sneezing, and coughing, ventilation, heartbeat, blood pressure.

• The gustatory nucleus in the medulla oblongata will then transmit the taste information to the thalamus which relays the information to the gustatory cortex for processing.

The thalamus is a part of the forebrain involved in the regulation of alertness, sleep, and consciousness.

It is also important to add that gustation (taste) is strongly linked to olfaction (smell) as the flavor of any type of food requires a combination of the two senses.

1. What Are the 5 Types of Taste?

The chemical substances that are generated during the breakdown can be classified as sweet, salty, sour, bitter, and umami [2]. However, some foods may have a mixture of more than 2 types of taste.

Sweet

A sweet taste is due to the presence of sugars such as glucose, fructose, or artificial sweeteners including aspartame, sucralose, or saccharine.

Salty

A salty taste is due to the presence of sodium chloride and other salts.

Sour

A sour taste is due to the presence of acids such as citric acid (orange juice)

Bitter

A bitter taste is the opposite taste to the acidic taste and is due to the presence of alkaloids that are mostly found in plants such as tea, coffee, aspirin, and tannins.

Umami

Umami taste is due to the presence of amino acids that form proteins found in meaty products.

Fatty taste is another type that has been proposed as a specific receptor for the omega 3 fatty acid, linoleic acid, was identified. Linoleic acid is mainly found in plant oils.

In addition, hot or spicy are not considered as tastes as they are pain signals associated with touch and temperature.

Taste qualities are also associated with the ability to distinguish between pleasant and unpleasant tastes but also between poison from food.

For example, most people tend to prefer sweet and umami tastes, while sour and bitter tastes are less preferred.

2. How Many Taste Buds Do Humans Have?

Humans have between 2000 to 4000 buds in total and are found in the papillae of the tongue but also in the back of the throat, nasal cavity, epiglottis, and the upper part of the esophagus. They are renewed every week [1].

Each taste bud contains between 10-40 gustatory receptors (sensory cells).

Papillae are divided into fungiform papillae (200-400 papillae), circumvallate papillae (7-12 papillae), and foliate papillae (about 20 papillae).

3. What Is a Flavor?

A flavor is a perception of food that combines taste and smell. It is mediated by the trigeminal nerve responsible for facial sensations, biting, and chewing.

4. What Are the Main Components of Flavor?

The main components of flavor include:

• Tastes (Sweet, bitter, acid, umami, and salty)
• Aroma (Pleasant smell from consumed food)
• Trigeminal Sensations (Facial sensations mediated by the trigeminal nerve)

6. What Part of the Brain Controls the Tongue?

The muscles monitoring the movement of the tongue are controlled by the medulla oblongata through the hypoglossal nerve.

7. What Causes a Person to Lose Taste?

The loss of taste is known as ageusia that can be due to several causes [3]:

• Head injury
• Middle ears or upper respiratory infections
• Dental problems or inadequate oral hygiene
• Environmental exposure to chemicals
• Medications
• Head and neck cancer radiotherapy
• Some surgical procedures involving the nose, ear, or the throat.

Disorders of taste can manifest in the following forms:

• Ageusia is disorder characterized by the complete absence of taste.
• Hypogeusia is adiminished taste.
• Hypergeusia is characterized by an enhanced perception of taste.
• Dysgeusia or parageusia is a distortion of tastemainly associated with an unpleasant perception of taste.

8- What are the symptoms of Taste Disorders?

• Changes in the taste of food and beverages that are usually consumed
• Changes in the intensity of taste of consumed food and beverages
• Absence or loss of taste
• Loss of weight associated with a loss of appetite

8- How are Taste Disorders Diagnosed

The otolaryngologist can assess the taste disorder by measuring the capacity of the affected individual to taste different types of tastes [3].

As taste disorders can be associated with middle ear and upper respiratory infections, the otolaryngologist will also examine potential problems with the ears and the throat.

For potential causes associated with poor dental hygiene, a dental examination is also performed.

8. How Can I Improve My Sense of Taste?

Treatment and improvement of the sense of taste depend on the cause of the taste disorder.

 however, some of these disorders can be prevented by ensuring good oral hygiene, consuming a variety of foods with different flavors, and staying patient when affected by cold.

Conclusion

Taste is very important for our everyday life. Not being able to taste a good dish of food or a beverage can be extremely stressful, and in some cases, can lead to a loss of appetite, weight loss, and depression. Unfortunately, it is only when we are having issues with our taste that we remember how important it is in our everyday life.

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1. Physiological Mechanisms of Headaches and Migraines

Migraine can be due to a genetic complex familial disorder that is most likely governed by several genes, however, both headache and migraine are caused by trigeminal nerve activation and calcitonin gene-related peptide (CGRP) release from the trigeminovascular system.

The trigeminal nerve transmits sensations of touch and pain in the face and motor functions. These sensations are transmitted through the nerve’s three major branches: the ophthalmic, maxillary, and mandibular nerves [1].

Calcitonin gene-related peptide (CGRP) is a neuropeptide produced and secreted by the thyroid C cells and stored in the trigeminovascular system, which consists of a complex of sensory neurons that innervate cerebral blood vessels, pial vessels, dura mater, and large venous sinuses.

Once released from these neurons, CGRP has been associated with nociceptive transmission (perception or sensation of pain), and release from perivascular nerve endings causing neurogenic vasodilatation [2]. 

2. Causes of Headaches and Migraines

Several causes trigger a headache and migraine, and among these, stress, environment, diet, hormones, lack of sleep, and gallbladder disorders, are the main ones.

A. Stress and Migraines

Migraines and headaches are often caused by stress. When a person is stressed, the body releases certain hormones which can trigger migraines and headaches. These hormones can cause the blood vessels in the head to dilate, leading to pain [3].

Additionally, stress can lead to tension headaches, which are caused by muscle contractions in the neck and head. Tension headaches are the most common type of headache, and they can be triggered by stress, fatigue, poor posture, and eyestrain.

B. Diet and Migraines

Many experts believe that diet may play a role. Some foods are known to trigger migraines and headaches, including aged cheeses, processed meats, nuts, and chocolate. Drinking too much alcohol or caffeine can also bring on these unpleasant symptoms [4].

If you suffer from migraines or headaches, it’s important to keep track of what foods seem to aggravate them. Try eliminating suspected triggers from your diet for a few weeks and see if your symptoms improve. You may also want to try eating smaller meals more often throughout the day instead of three large meals. Getting enough sleep and regular exercise can also help reduce the likelihood of migraines and headaches.

C. Gall Bladder and Migraines

The association between the gall bladder and migraines has been shown by a study of 20,427 patients who were diagnosed with gallbladder stone disease (GSD) between 2000 and 2011 from Taiwan’s National Health Insurance Research Database (NHIRD). The study reported a greater cumulative incidence of migraines in patients with GSD compared with those without GSD [5].  

D. Environmental Factors and Migraines

Environmental factors that cause headaches and migraines are associated with seasonal allergies and weather changes that can affect the sinuses.

E. Hormones and Migraines

A link between hormones such as estrogens and migraines has also been reported. This is reflected by the increased frequency of migraines during menstruation [6].

F. Sleep and Migraines

Finally, a lack of sleep is another cause of headaches and migraines. This interaction was shown by a study of 50 migraineurs which confirmed previous observations that sleep is a common way of ending the headache phase [7].

3. Headaches and Migraines treatment

Migraines can be debilitating and frustrating. Many people do not know how to treat them. There are many treatments for migraines, but what works for one person may not work for another. Some people find that over-the-counter medications work well (e.g., Ibuprofen or Aspirin), while others need prescription medication.

Some people find that they need to see a neurologist to get help with their migraines. Several alternative treatments can help, including acupuncture and chiropractic care.

4. Frequently Asked Questions About What Causes Headaches and Migraines

What causes headaches and migraines?

Headaches and migraines can be caused by various factors, including stress, tension, dehydration, lack of sleep, hormonal changes, certain foods, environmental factors, and underlying health conditions.

Can weather changes cause headaches and migraines?

Yes, some people are sensitive to changes in weather patterns, such as barometric pressure changes, which can trigger headaches and migraines.

Are headaches and migraines hereditary?

There is evidence to suggest that genetics play a role in predisposing individuals to migraines. If one or both parents have a history of migraines, there’s an increased likelihood of experiencing them.

Can food and drinks trigger headaches and migraines?

Yes, certain foods and drinks, such as aged cheeses, processed meats, caffeine, alcohol, and artificial sweeteners, can trigger headaches and migraines in susceptible individuals.

Can stress cause headaches and migraines?

Yes, stress is a common trigger for both tension headaches and migraines. Managing stress through relaxation techniques, exercise, and mindfulness can help reduce the frequency and severity of headaches.

Are there any specific environmental factors that can trigger headaches and migraines?

Yes, environmental factors such as bright lights, loud noises, strong odors, and changes in altitude or air pressure can trigger headaches and migraines in some people.

Can hormonal changes trigger headaches and migraines?

Yes, hormonal fluctuations, such as those occurring during menstruation, pregnancy, or menopause, can trigger migraines in some individuals.

Are there certain medical conditions that can cause headaches and migraines?

Yes, underlying medical conditions such as high blood pressure, sinus infections, temporomandibular joint (TMJ) disorder, and intracranial abnormalities can cause headaches and migraines.

How can I determine what triggers my headaches and migraines?

Keeping a headache diary can help identify triggers by tracking factors such as food and drink intake, sleep patterns, stress levels, weather changes, and menstrual cycles, along with headache occurrence.

When should I see a doctor about my headaches and migraines?

You should see a doctor if you experience severe or frequent headaches, headaches that worsen over time, headaches accompanied by other symptoms such as vision changes, numbness, weakness, or difficulty speaking, or if your headaches interfere with daily activities despite over-the-counter treatments.

Conclusion

Although some headaches and migraines appear to be due to a hereditary genetic complex familial disorder, most are due to stress and diet that can be individually controllable through a healthy lifestyle.

Other types of headaches and migraines are periodic due to menstruation in women and environmental changes. However, treatment for gallbladder stone disease is required for related headaches and migraines.

References

[1] https://teachmeanatomy.info/head/cranial-nerves/trigeminal-nerve/

[2] Benemei, S., Nicoletti, P., Capone, J.G. and Geppetti, P., 2009. CGRP receptors in the control of pain and inflammation. Current opinion in pharmacology, 9(1), pp.9-14.

[3] Sauro, K.M. and Becker, W.J., 2009. The stress and migraine interaction. Headache: The journal of head and face pain, 49(9), pp.1378-1386.

[4] Finocchi, C. and Sivori, G., 2012. Food as trigger and aggravating factor of migraine. Neurological Sciences, 33(1), pp.77-80.

[5] Chen, C.H., Lin, C.L. and Kao, C.H., 2018. Gallbladder stone disease is associated with an increased risk of migraines. Journal of clinical medicine, 7(11), p.455.

[6] MacGregor, A., 2000. Migraine associated with menstruation. Functional neurology, 15, pp.143-153.

[7] Blau, J.N., 1982. Resolution of migraine attacks: sleep and the recovery phase. Journal of Neurology, Neurosurgery & Psychiatry, 45(3), pp.223-226.

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